As pointed out in my copending and commonly assigned U.S. patent application, which was filed Dec. 28, 1989 under Ser. No. 07/459059 on "Transmitted Code Clock Code-Matching Synchronization for Spread-Spectrum Communication Systems" (D/89523) and which hereby is incorporated by reference, spread-spectrum communication systems are superior to ordinary narrow band systems in several important respects. For example, they have greater immunity to narrow band noise, and they are less likely to cause narrow band interference. Furthermore, they provide increased "unencrypted" security against unauthorized eavesdropping because conventional narrow band signal detectors are illsuited for separating spread-spectrum signals from the usual background or "floor" noise.
Frequency bands of 902 MHz-928 MHz, 2400 MHz-2483.5 MHz and 5725 MHZ-5850 MHz were allocated a few years ago for license-free spread-spectrum communications at transmitted power levels of up to one (1) watt, subject to certain restrictions on the distribution of the sideband energy. That action is likely to promote the commercial use of this technology for short range radio communications.
To carry out so-called "direct sequence" spread-spectrum communications, a transmitter characteristically mixes a cyclical pseudo-random code sequence with an information modulated carrier signal, thereby "spreading" the spectrum of the transmitted signal energy generally uniformly across a wide band of frequencies. The transmitter can utilize any of several well known modulation techniques for impressing baseband information on the carrier, including frequency modulation (FM), frequency shift keying (FSK), phase modulation (PM), and phase shift keying (PSK). For recovering the baseband information from a incoming spread-spectrum signal of the foregoing type, a receiver first mixes the incoming signal with a locally derived or a locally generated pseudo-random code sequence which is substantially synchronized with the transmitted code sequence, thereby "despreading" the signal spectrum to recover the carrier. A suitable demodulator then demodulates the carrier to recover the baseband signal.
Various synchronization processes have been developed for synchronizing a code generator residing at a receiver of such a system with the code generator at the transmitter. However, the following discussion is directed toward so-called "carrier lock tracking." As is known, carrier lock tracking is an attractive technique for synchronizing direct-sequence spread-spectrum communication systems, especially for applications in which it is necessary or desirable to utilize "code-division multiplexing" for sharing the available frequency spectrum among multiple user groups who might engage in time overlapping communications.
Carrier lock tracking-type synchronization is based on the premise that all receivers for which a given transmission is intended have a priori knowledge of the spectrum spreading code sequence for that particular transmission. In keeping with that premise, the transmitter and each of the participating receivers are equipped with respective clock-driven pseudo-random code generators which generate essentially identical pulse code sequences. At the outset of a communication session, the phase relationship between the code sequence that is being locally generated at a given receiver and the transmitted code sequence arriving at that receiver is arbitrary and undefined. For that reason, each of the receivers typically is initialized by a sliding correlator which phase aligns its locally generated code sequence with the transmitted code sequence. More particularly, until the receiver determines that its locally generated code sequence is phase aligned with the transmitted code sequence, its local code generator is clocked at a frequency which is offset slightly from the frequency at which the transmitted code sequence is clocked. This causes the relative phase of the transmitted and locally generated code sequences to vary, preferably at a rate which is sufficiently slow to enable their phase alignment to be detected within the time required for their relative phase to shift by a single code bit (i.e., the time span of the so-called "correlation window"). When such a phase correlation is detected, the receiver adjusts the clock frequency for its local code generator, thereby causing it to be synchronously clocked at essentially the same frequency as the transmitted code sequence for the remainder of the communication session.
To implement carrier lock tracking, the clock frequency for the transmitted code sequence usually is selected to be a submultiple of the carrier frequency, whereby each the receivers can utilize a suitable frequency divider for deriving the synchronous clock frequency for its local code generator from the carrier signal it recovers. In other words, each of the receivers typically relies upon a sliding correlation process for finding "carrier lock," and a carrier detection/frequency division process for maintaining lock. Unfortunately, however, this tends to cause the receivers to consume substantial amounts of power, especially in systems which are designed to operate at high frequencies, such as at the UHF frequencies which have been allocated for license-free spread-spectrum radio communications. Clearly, power consumption is a significant issue, particularly for systems having battery powered receivers, such as might be found in portable computers having spread-spectrum communication links, because the receivers generally are powered-up more or less continuously to operate in a standby state pending the arrival of a transmission. Moreover, straightforward frequency division is feasible for recovering the synchronous clock frequency from the carrier only if there is a harmonic relationship between the clock frequency and the carrier frequency, which sometimes is an unattractive design constraint.
Accordingly, it will apparent that there is a need for methods and means for synchronizing direct-sequence spread-spectrum communication systems, including systems that are compatible with the use of code-division multiplexing, for applications in which it is necessary or desirable to significantly reduce the power consumption of the receivers. Moreover, it will be evident that it would be beneficial for designers to have the freedom to select the carrier frequencies and the code clock frequencies for such systems independently of each other. As will be seen, the invention described and claimed in my aforementioned copending and commonly assigned patent application addresses similar needs, but it will be evident that this invention and my prior invention provide significantly different responses to those needs.